Gaze for at least 10 s at the fixation cross in the neighbouring dynamic image. This will adapt your eyes to a brightening patch (above) and a repetitively dimming patch (below). Then click the mouse, or press the spacebar or tap the panel to turn both patches into a steady gray. You should see an aftereffect of apparent dimming (above) and apparent brightening (below) for a few seconds.

This demonstrates the existence of transient visual pathways selective for gradual changes of luminance.